Iron hydride complexes

Abstract Organic syntheses catalyzed by iron complexes have attracted considerable attention because iron is an abundant, inexpensive, and environmentally benign metal. It has been documented that various iron hydride complexes play important roles in catalytic cycles such as hydrogenation, hydrosilylation, hydro-boration, hydrogen generation, and element-element bond formation. This chapter summarizes the recent developments, mainly from 2000 to 2009, of iron catalysts involving hydride ligand(s) and the role of Fe-H species in catalytic cycles. 

Keywords Catalysis Electrochemical reduction Hydroboration Hydrogenation Hydrosilylation Iron hydride complex Photochemical reduction... 

In 1931, Hieber and Leutert reported Fe(CO)4(H)2 not only as the first iron hydride complex but also as the first transition-metal hydride complex (FeH2 was reported in 1929 from FeCl2 and PhMgBr under a hydrogen atmosphere. However, it exists only in a gas phase) [2, 3]. The complex synthesized from Fe(CO)5 and OH (Scheme 1) is isolable only at low temperature and decomposes at room temperature into Fe(CO)5, Fe(CO)3, and H2. 

Fig. 1 The first X-ray crystal structures of three types of the iron hydride complexes...

Iron hydride complexes can be synthesized by many routes. Some typical methods are listed in Scheme 2. Protonation of an anionic iron complex or substitution of hydride for one electron donor ligands, such as halides, affords hydride complexes. NaBH4 and L1A1H4 are generally used as the hydride source for the latter transformation. Oxidative addition of H2 and E-H to a low valent and unsaturated iron complex gives a hydride complex. Furthermore, p-hydride abstraction from an alkyl iron complex affords a hydride complex with olefin coordination. The last two reactions are frequently involved in catalytic cycles. 

A Fe-H bond is generally polarized as Fe -H because H is more electronegative than Fe. However, iron hydride complexes impart much less negative charge to the hydride than early transition-metal hydride complexes. 

The proposed mechanism for Fe-catalyzed 1,4-hydroboration is shown in Scheme 28. The FeCl2 is initially reduced by magnesium and then the 1,3-diene coordinates to the iron center (I II). The oxidative addition of the B-D bond of pinacolborane-tfi to II yields the iron hydride complex III. This species III undergoes a migratory insertion of the coordinated 1,3-diene into either the Fe-B bond to produce 7i-allyl hydride complex IV or the Fe-D bond to produce 7i-allyl boryl complex V. The ti-c rearrangement takes place (IV VI, V VII). Subsequently, reductive elimination to give the C-D bond from VI or to give the C-B bond from VII yields the deuterated hydroboration product and reinstalls an intermediate II to complete the catalytic cycle. However, up to date it has not been possible to confirm which pathway is correct. 

The isolated iron hydride complexes introduced in this chapter are listed in Table 12, where the hydride chemical shifts in the HNMR spectra and the Fe-H bond distances are summarized. 

The mechanism of the C—H and C—C bond activation of bare Fe+ with n-heptyltrimethylsilane has been elucidated with the help of extensive labeling studies71. The system was found to display a rather rich chemistry. Loss of neutral tetramethylsilane from the ion-molecule complex (equation 11) was explained by an initial insertion of the metal ion into the Cl— C2 bond to form 21, and a subsequent fi-H shift giving rise to the iron-hydride complex 22. This ion can then lose a tetramethylsilane molecule via reductive elimination. 

Since silene is an unstable species, various transition metal-silene complexes coordinated by the silicon-carbon double bond have been reported. In 1970, Pannel reported the formation of silene by irradiation of an iron complex (Eq. 6) [8]. He obtained an iron-TMS complex that was apparently formed from silene and an iron-hydride complex generated from the starting iron complex by /3-hydrogen elimination [8]. Wrighton confirmed the existence of tungsten-and iron-silene complexes by examination of NMR spectra obtained at low temperature (Eqs. 7 and 8) [9]. 

The ort o-metalated iron hydride complexes [HFe(CO)2 P(OPh)3 (PhO)2POC6H4 ] with R C=CR (R =Ph, R2 = Mc R = R2 = Mc R =Me, R2 = CH(OEt)2 R =Me, R = CH20H R = R = CH20H) in the presence of hydrated zinc chloride give a series of ferracyclopentendione complexes 203 <2000JOM(612)61>. With phenylace-tylene under similar conditions, the ferrole-type species 204 is formed with structural parameters similar to those... 

Hundreds of iron hydride complexes with phosphine ligands have been isolated and characterized. These complexes are almost always diamagnetic. For example, a spectacularly active electrocatalyst for dihydrogen oxidation is the cyclopentadienyl iron... 

A few studies of isolated metal-silyl complexes and ttie computational study of rhodium-sUyl complexes illustrate the insertion of olefins into metal-silicon bonds. Wrighton studied the photochemical reaction of iron-silyl complexes witti ettiylene (Scheme 9.13). Photolysis of Cp FefCOl fSiMej) in the presence of ettiylene forms Cp Fe(CO)(CjHJ(SiMej). This complex appears to insert ethylene, but ttie 16-electron insertion product is unstable and forms the corresponding vinylsilane and iron hydride complexes as products. Photolysis of Cp Fe(CO)j(SiMe3) in the presence of ethylene and CO forms ttie p-silylaDcyl complex containing two CO ligands. 

If the coordination sphere is protected by bulky ligands such as Cp and dppe, the hydride complexes can exist in different oxidation states, which enhances its reactivity. Thus, Hamon has isolated and studied 17-, 18- and 19-electron iron hydride complexes in various oxidation states ... 

Square-redox scheme for the CO-induced reductive elimination of hydride from an iron-hydride complex... 

The protonation of the iron-hydride complex [Fe Cp(PMe3)2(H)] leads to the H2 complex [Fe Cp(PMe3)2(H2)]+, because the iron atom in the starting complex is relatively electron poor, and the oxidation state Fe is much more favorable than Fe. On the other hand, the protonation of the osmium complex leads to the protonation of that is an electron-rich metal center, i.e. the oxidative addition occurs to yield the dihydride [Os Cp(PMe3)2(H)2], the oxidation state Os being much more common than Fe. ... 

Alkyl isocyanates insert into tungsten and iron hydride complexes to give the insertion products resulting from insertion into the metal-hydrogen bond. For example, reaction of CpW(CO)3H with methyl isocyanate affords the carbamate complex . Likewise, CpFe(CO)2H reacts with t-butyl isocyanate to give the carbamate complex 329 . 

A well-defined iron hydride complex FeH(CO)(NO)(Ph3P)2 is highly active as a catalyst for selective hydrosilylation of internal alkynes to vinylsilanes. Depending on the silane employed, either E- or Z-selective hydrosilylation products are formed in excellent yields and good to excellent stereoselectivities. The stereochemical course of this transformation is dependent on the steric demand of the substituents on the silane. " A new 0 family of Lewis-basic 2-pyridyloxazolines catalyses the enantioselective reduction of prochiral aromatic ketones and ketimines using trichlorosilane. 1-Isoquinolyloxazoline (20) derivative was identified as the most efficient catalyst of the series capable of delivering high enantioselectivities in the reduction of both ketones (up to 94% ee) and ketimines (up to 89% (p)... 

The iron hydride complex FeH(CO)(NO)(Ph3P)2 has been reported to catalyse selective hydrosilylation of internal alkynes Ar C=CAr with PhSiHj. The corresponding intermediate (Z)-vinylsilanes Ar CH=C(SiH2Ph)Ar thus generated then produce trans-Ar CH=CHAr. With PhMeSi(H)CH=CH2 as the reagent, di-Ar CH=CHAr were obtained. Mechanistic details of this stereodivergent method have been diseussed. Tetrahydropyrans and piperidines (218) were obtained by the Fe -catalysed intramolecular hydroalkoxylation and hydroamination of allenes (217). ... 

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